Optical devices that include a diffraction grating are known in the field of optoelectronics, such as certain types of optical filters, optical couplers, distributed-feedback (DFB) semiconductor lasers, and distributed Bragg reflector (DBR) lasers.
In wavelength controlling semiconductor lasers or wavelength-tunable semiconductor lasers that have a diffraction grating, such as DFB and DBR semiconductor lasers, for example, parameters such as the period, shape and depth of the diffraction grating greatly affect laser characteristics such as the coupling coefficient and lasing threshold. Therefore, it is important to fabricate a precise diffraction grating in a highly controllable manner.
A commonly used method for fabricating diffraction gratings involves two steps: forming a photoresist mask patterned in the shape of a grating and etching the regions that are not covered with the mask.
Widely used methods for forming the grating pattern photoresist are electron beam (EB) exposure and holographic exposure.
To expose a photoresist by the EB exposure, first the photoresist is applied onto a silicon oxide film formed on a compound semiconductor film and is baked. Then the photoresist is irradiated with an electron beam to form a striped grating latent image having a period Λ on the photoresist.
The electron beam used in the EB exposure is produced by focusing a stream of electrons emitted from an electron gun through the use of a lens. The shape, size and deflection position are controlled with optics such as a deflector, electron lens, and aperture.
To expose a photoresist by the holographic exposure, first the photoresist is applied onto a silicon oxide film formed on a compound semiconductor film and is baked. Then the photoresist is irradiated with laser beams from two directions to form a latent image of a holographic pattern.
The period Λ of the holographic pattern is Λ=λ/2 sin θ, where θ is the incident angle of the two laser beams and λ is the wavelength. For example, an Ar laser having a wavelength λ of 351 or a He—Cd laser having a wavelength λ of 325 nm is used for the exposure. With such a laser, a first-order diffraction grating used in a DFB semiconductor laser having an emission wavelength in a 1.3 μm band or of 1.55 μm may be readily and reproductively fabricated.
The latent image of the photoresist formed by any of these exposure methods is developed to form a visible image, which is used as resist mask having a grating pattern. Regions in the silicon oxide film that are exposed in the resist mask are etched into a grating pattern by dry etching, for example.
The grating-patterned silicon oxide film is used as a mask in etching of the compound semiconductor film. Regions of the compound semiconductor film that are exposed in the grating-patterned silicon oxide film are etched to form a diffraction grating having periodic grooves and ridges having a period Λ.
Recently compact and high modulation red, green and blue lasers are needed for micro projection displays. Semiconductor lasers of red and blue wavelength bands among the three primary color bands are commercially available, but a semiconductor laser that emits light of a green wavelength band, for example a wavelength of 532 nm, has not been made commercially available.
Therefore, a device including an excitation light source of a wavelength in a 1.06 μm band and a second harmonic generation (SHG) device is commonly used as a green-light emitter. In that case, a DFB or DBR semiconductor laser, for example, is used as the excitation light source. Fundamental laser light emitted from such a semiconductor laser is converted to laser light having a ½ wavelength through the SHG device.
The period Λ of a first-order diffraction grating formed in a waveguide of the DFB or DBR semiconductor laser used in the green light emitter is 158 nm.
EB exposure may be used to form the diffraction grating having a period Λ of 158 nm. However, the EB exposure involves scanning with an electron beam as many times as the number of rulings of the diffraction grating, requesting long exposure time per substrate. Therefore, a diffraction grating fabrication method that uses the EB exposure does not lend itself to volume production.
In contrast, the holographic exposure requests a short exposure time. However, the period Λ of the diffraction grating that may be fabricated is restricted by the wavelength of laser light used for exposure and is also affected by the numerical aperture (NA) of the optical system of exposure equipment. The period Λ of repetition of grooves and ridges of the first-order diffraction grating that may be fabricated is 175 nm at the shortest. Therefore, it is difficult to form a latent image of a first-order diffraction grating having a period of approximately 158 nm by the holographic exposure.
It may be contemplated to fabricate a second or higher order diffraction grating by holographic exposure. However, a higher-order diffraction grating has a larger light scattering loss and therefore lower emission efficiency. Therefore it is difficult to improve laser characteristics with holographic exposure. As is appreciated, a second-order diffraction grating has a period 2Λ twice the period Λ of a first-order grating.
There is a known method for fabricating a first-order diffraction grating constituting a short-wavelength DFB or DBR semiconductor laser made of a GaAs-based material as described below.
A holographic exposure method and a process such as wet etching with a H2SO4-based etchant are used to form a second-order diffraction grating having a period 2Λ on an optical waveguide made of p-AlGaAs.
Then a light absorbing layer is formed on multiple ridges in the diffraction grating while at the same time a light absorbing layer is formed on the bottom of grooves between the ridges. The light absorbing layers are used as a first-order diffraction grating having a period Λ.    [Patent document 1] Japanese Laid-open Patent Publication No. 2008-218996    [Patent document 2] Japanese Laid-open Patent Publication No. 2000-019316    [Patent document 3] Japanese Laid-open Patent Publication No. 09-186394